A so-called passive type automatic focusing system creates an image of a subject on a CCD image sensor, determines the degree of blur of the image of the subject based on a luminance signal output from the CCD sensor, indicates the luminance of the image of the subject and derives the defocused amount thereof. Then, it effects the focusing in order to eliminate the defocusing amount.
As a CCD image sensor used in the above system, a sensor including a shift register disposed on one side of a large number of photoelectric elements (which are hereinafter referred to as "picture elements") arranged in a preset direction and connected to odd-numbered picture elements and a shift register disposed on the other side of the picture elements and connected to the even-numbered picture elements is known.
For example, a CCD line image sensor constructed as shown in FIG. 5 is used in the above system.
In the image sensor 1, a light receiving section 2 constructed by a plurality of picture elements is formed in a line form and two analog shift registers 3 and 4 are arranged along the light receiving section 2.
In the CCD image sensor 1, odd-numbered picture elements are connected to the shift register 3 and even-numbered picture elements are connected to the shift register 4.
Charges read out from the picture elements of the light receiving section 2 are temporarily stored in corresponding memories of the shift registers 3 and 4 and then sequentially transferred to a charge storage section 5 (since charges stored in the charge storage section are dealt as an output of the CCD image sensor, the charge storage section is hereinafter referred to as an output section) which is formed of a capacitor in response to charge transfer clocks CK1 and CK2 shown in FIG. 6.
For example, charges stored in a picture element 2n are read out into the shift register 4 and transferred from the shift register 4 to the output section 5 in response to the charge transfer clocks CK1 and CK2. A potential corresponding to the amount of charges stored in the output section 5 appears on the output side of the output section 5.
The output side of the output section 5 is connected to a sampling circuit (not shown). The sampling circuit samples the output side potential of the output section to receive the potential as a luminance signal before a reset pulse is applied to the output section 5.
In the timing chart shown in FIG. 6, a reset pulse is applied to the output section 5 so as to permit the charges of the picture element 2n to be discharged from the output section 5 before the charge transfer clocks CK1 and CK2 are changed. Then, charges of a picture element (2n-1) which are transferred by the charge transfer clocks CK1 and CK2 are stored into the thus reset output section 5.
Next, in the same manner as in a case of the picture element 2n, charges of the picture element (2n-1) stored in the output section 5 are sampled by the sampling circuit. After this, the output section 5 is reset by the reset pulse.
Thus, each time charges of one picture element are stored in the output section 5, the output section is reset and the charges read out from the picture element are sampled for each picture element.
Since a luminance signal derived from the sampling circuit contains defocusing information, the focusing position can be derived based on the luminance signal. Various methods are known as a method of deriving the focusing position by use of the luminance signal.
In a case where the amount of light of an image of a subject formed on the CCD image sensor is small, it is necessary to set the charge storage time of the light receiving section 2 longer in order to prevent degradation of the sensitivity and S/N ratio. If the charge storage time of the light receiving section 2 is set longer, the amount of charges stored in each picture element increases to improve the sensitivity and S/N ratio, thereby making it possible to attain high focusing position detecting precision.
However, with the above method, since the charge storage time of the light receiving section 2 is set longer, a problem that the focusing position detecting time will become longer occurs.